A stepping motor is controlled by alternatingly energizing its windings. In the type known from EP 0 394 941 A2 FIG. 2 shows a typical control sequence for the half-step mode preferred for its quiet running for a two-phase stepping motor with two windings. FIG. 2(a) shows bipolar current pulse sequence IL1 for one winding. FIG. 2(b) shows bipolar current pulse sequence IL2 for the other winding. The armature position of the stepping motor resulting from this control sequence for half-step mode is indicated in FIG. 2(c).
The periodic change-over of the pulsed winding currents causes electromagnetic interference to occur both over the motor leads connected with a motor driver and over the supply voltage lines. These lines thereby act as antennas. By making the leads from the driver to the motor very short one can practically eliminate their antenna effect. However the supply voltage lines are relatively long and form very good antennas.
The driver circuit known from EP 0 394 941 A2 performs current regulation even during maximum current flow. This leads to a corresponding voltage drop on a regulating transistor. The consequence is a high power loss.
From DE 29 44 335 A1 it is known to supply the windings of a stepping motor with current pulses which are not digital but have a staircase form with four amplitude values. This makes it possible to increase the step number per revolution of the stepping motor. The resulting total current contains stages which lead to interference.
The driver circuit known from DE 29 44 335 A1 works in principle like the driver circuit in EP 0 394 941 A2 but uses a clocked regulation with pulse width modulation. Although this reduces losses, it increases the interference.
Known stepping motors include so-called chopping or clocked stepping motor drivers e.g. as in DE 29 44 335 A1, which subject the windings of the stepping motor to a series of pulses during each energizing phase, and non-chopping stepping motor drivers (e.g. as in EP 0 394 941 A2), which subject the individual windings to only one current pulse during each energizing phase.
With chopping stepping motor drivers, current regulation is effected by pulse width modulation of the voltage across the particular motor winding energized. With the switching frequencies of a few 10 kHz usual today, considerable electromagnetic interference occurs unless suitable blocking measures are taken. A relevant part of the total design effort and financial expense for a stepping motor system with such current regulation is required for these blocking measures. A typical integrated stepping motor driver with chopping current regulation is PBL 3717 from SGS-Thomson, shown in the corresponding data sheet.
With non-chopping stepping motor drivers, current adjustment is effected via the ohmic resistance of each motor winding. The chopper disturbances due to the high switching frequencies mentioned in connection with chopping stepping motor drivers do not exist in chopping stepping motor drivers. However the current pulses, as are shown in FIGS. 2(a) and 2(b), still cause interference in the case of steep pulse edges. Interference over the leads from the stepping motor driver to the motor windings can be very greatly reduced, as already mentioned, by keeping the leads from the stepping motor driver to the motor windings very short so that these leads no longer act as antennas. However, total current Ig which the system comprising stepping motor and stepping motor driver draws from the supply voltage source via supply voltage lines still contains a large alternating component with four times the frequency of the drive sequence. This is shown in FIG. 3. At the times when both motor windings are energized the current consumption of the system is twice as high as at the times when only one motor winding is energized. This has the effect of pulses being superimposed on the low current consumption value. This leads to interference over the supply voltage lines.
In some applications one attempts to reduce the interference over the motor leads and supply voltage lines by providing accordingly slow switching edges of the stepping motor driver. This eases the problem of interference but does not fundamentally eliminate it.
An example of this technology is the integrated stepping motor driver MC 33192 from Motorola, shown in the corresponding data sheet.
Operating one of the two driver elements in each bridge arm, i.e. each driver element series connection, of a bridge circuit as a controllable switch and the other driver element as a controllable amplifier element is known in the art from the abovementioned prints EP 0 394 941 A2 and DE 29 44 355 A1.
In EP 0 394 941 A2 this measure serves to compensate winding unevenness which would lead to step errors of the stepping motor if not compensated. To overcome this, one of the two driver elements is formed by the transistor of a current mirror via which the constant current from a constant current source is impressed on this driver element when the latter is rendered conductive along with a switching transistor of the other bridge arm of the full bridge.
In DE 29 44 335 A1, as already mentioned, a subdivision of the step size and thus an increase in step number per revolution is effected by staircase pulses. For this purpose one driver element of each bridge arm is formed by a controllable transistor which is switched on and off with the help of a flip-flop and is fed in the on state, via the output of a comparator whose one input is fed the output signal from a current sensor resistor measuring the winding current and whose other input is fed a reference voltage of variable voltage determined by a constant-value storage, a control voltage which leads to a winding current with an instantaneous stage value corresponding to the storage value released instantaneously by the constant-value storage.
EP 0 083 841 A1 discloses stepping motor control means for operating a stepping motor either in step mode or in linear mode. Each of two motor windings has associated therewith a full bridge circuit with two bridge arms each having two switching transistors in series connection. Associated with each winding is a current sensor resistor whose voltage can be divided down to one of several possible component voltages by means of a voltage divider in accordance with the readout value of a constant-value storage. This component voltage is compared in a comparator with a reference voltage coming from a triangle generator. Using phase driver circuits which are fed an output value from the constant-value storage on the one hand and the output signal from the comparator on the other hand, one can drive the switching transistors of the full bridge circuit to conduct winding currents of different current step heights. In linear mode of the stepping motor this stepped drive of the switching transistors is to lead to a constant total current through the two motor windings. However, since no regulation is involved here it is not ensured that a constant total current is actually reached under all operating conditions.
From DE 36 10 253 C2 it is known in the art to provide an edge steepness reducing circuit for reducing the steepness of the driver control pulses in connection with a control circuit for a DC motor with no commutator.